CODEWITHSUNDEEP
python
user2321808
user2321808

Reputation:

Python array -- elements replaced with last value

Given the code 

  def sumset(a, b):
    bands=[[0, 0]]*len(a)*len(b)
    current=-1
    for ba in a:
      for bb in b:
        current+=1
        bands[current][0]=ba[0]+bb[0]
        bands[current][1]=ba[1]+bb[1]
        print(bands[current])

    print(bands)

The output of sumset([[1,2], [2,4]], [[0,1], [8, 9]]) gives

[1, 3]
[9, 11]
[2, 5]
[10, 13]
[[10, 13], [10, 13], [10, 13], [10, 13]]

I cannot understand why bands is filled with bands[3].

EDIT: I am using Python 3.4.2 on Ubuntu 14.10.

Upvotes: 0

Views: 48

Answers (1)

yedpodtrzitko
yedpodtrzitko

Reputation: 9359

By multiplying a list you're creating a copies of the initial list. So when you modify one of them, the changes are transfered into the other copies as well.

Upvotes: 1

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